The lonely planet phrase book is pretty darn good. A few tips though, make sure you learn some basic phrases, don't read them out. Whenever possible try and at least attempt to speak a little Japanese, when I was in Tokyo I tried to use as little English as possible (which wasnt hard because its hardly spoken) and the locals really appreciate it. Don't be afraid of mispronounciation just try your best!! Although that said, every time I got confused looking at a map someone was there in a few seconds asking me if I was ok and taking an excuse to practise their English. Seriously everyne was so helpful, welcoming and polite!! Last tip, the best phrase you can learn is excuse me/sorry!!!! Also I hope you have just as an amazing time as I had

Jyuuhassai and Jyuuhachisai is both correct. I personally say Jyuuhassai but then I speak quite a masculine sort of japanese unless I have to use keigo. As Adziu said the latter is more polite and "proper" way of pronouncing it. If one was nitpicky that sentence actually says the girl IS still 18. (Adult is 21 in Jp.)

And as for places depends on what youa re after, and when you are going. If you like manga and stuff isn't comiket soon?